1. Field of the Invention
The present invention relates generally to the field of batteries and, more specifically, to batteries in which mat containing glass fibers, commonly called separators, are positioned between the positive and negative plates and to a method for producing such mats or separators and batteries. As is subsequently discussed in more detail, separators containing glass fibers are well known. Long before glass fiber separators, however, cedar veneers were used as a separator material, and were replaced by microporous, hard rubbery separators and cellulose separators impregnated with resins.
2. Description of the Prior Art
Valve regulated ("sealed"--"recombinant") lead acid (VRLA) batteries are known; they usually comprise a plurality of positive and negative plates, as in a prismatic cell which can be a prismatic flat plate, or in layers of separator and positive and negative electrodes wound together, as in a "jelly roll" cell. The plates are arranged so that they alternate, negative - positive - negative, etc., with separator material separating each plate from adjacent plates. The separator, typically composed of a mat of wet laid nonwoven glass fibers, is an inert material; it stores battery acid, and provides low electric resistance. In addition, in VRLA batteries, the separator material provides innumerable gas channels between the plates through which oxygen can migrate from the positive electrode, when generated there, to the negative electrode where it can be recombined with hydrogen, according to the oxygen cycle. Another important function of a separator is to exert pressure against the plate paste or active material which forces the paste into contact with the plate, and causes a pressure between the positive active material and the positive grid and between the plates, ensuring that there is not an interface at which corrosion. which would cause premature capacity loss (PCL), can occur.
Glass fiber separator material has been produced commercially by wet processes on paper making equipment including fourdrinier machines and rotoformers, inclined fourdrinier machines and extended wire rotoformers. In the production of separator made of glass fibers for VRLA batteries, it is preferred that no organic binder be added to a furnish from which separator sheets are made; the entanglement of individual microglass fibers serves to maintain the sheet in a cohesive structure, and water glass or any of various sulfate salts, which sometimes form on the fiber surfaces, serves as a binder. Organic binders, however, tend to decrease the ability of a separator to wick acid, and to decrease the amount of acid a separator can hold. A great deal of work has been directed to modifying the glass fiber furnish from which separators are produced to improve battery performance and/or lower the cost of the separator. Some of the work has entailed the addition of synthetic fibers for various reasons, such as the use of thermoformable plastic fibers so that the separator can be heat sealed on its edges to envelop a plate. Other work, which pertains to the field of this invention, has been directed to the use of a filler, e.g., silica, or another siciciferous material, to provide separators which are comparable to all glass fiber separators, at a lower cost. Separators made from glass fibers to which cellulose has been added and polyolefin fibers to which cellulose has been added have also been suggested. Prior art patents are discussed below.
U.S. Pat. No. 4,465,748 (Harris) discloses glass fiber sheet material for use as a separator in an electrochemical cell, and made from 5 to 35 percent w/w of glass fibers less than 1 .mu.m in diameter; the patent also discloses a glass fiber sheet for such use wherein there are fibers of a continuous range of fiber diameters and lengths, and most of the fibers are not over 5 mm in length.
U.S. Pat. No. 4,216,280, (Kono et al.), discloses glass fiber sheet material for use as a plate separator in a battery, and made from 50 to 95 percent w/w of glass fibers less than 1 .mu.m in diameter and 50 to 5 percent w/w of coarser glass fibers. The coarser glass fibers, the reference says, have a fiber diameter larger than 5 .mu.m, preferably larger than 10 .mu.m , and it is advantageous for some of the coarser fibers to have diameters of 10 .mu.m to 30 .mu.m.
U.S. Pat. No. 4,205,122 (Minra et al.) discloses a battery separator of reduced electric resistance comprising a self supporting, non woven mat consisting essentially of a mixture of olefinic resin fibers having a coarseness of from 4 to 13 decigrex and olefinic resin fibers having a coarseness of less than 4 decigrex, the latter fibers being present in an amount of not less than 3 parts by weight per 100 parts by weight of fibers; up to about 600 parts by weight of inert filler materials per 100 parts by weight of fibers can also be used. The battery separator is produced by subjecting a suitable aqueous dispersion to a sheet-forming operation, drying the resulting wet, non-woven mat, and heat treating the dried mat at a temperature ranging from a point 20.degree. C. lower than the melting point of the aforementioned fibers to a point about 50.degree. C. higher than the melting point.
U.S. Pat. No. 4,216,281 (O'Rell et al.) discloses a separator material produced from a furnish containing 30 to 70 percent w/w of polyolefin synthetic pulp, 15 to 65 percent w/w of a siliceous filler and 1 to 35 percent w/w of "long" fibers which can be polyester fibers, glass fibers, or a mixture of the two. Cellulose in an amount up to about 10 percent w/w is disclosed as an optional ingredient of the furnish.
U.S. Pat. No. 4,336,314 (Yonnezu, et al), assigned to Japan Storage Battery Company, discloses a pasted lead acid battery with greatly extended service life and capacity over the usuable service life therof. The battery has a glass mat, which may be of a dual layer construction, disposed adjacent positive plates of assembled elements. This patent teaches the importance of pressure that must be applied to the assembled elements, for example by a binding band, or from outside the battery container. As the pressure applied to the elements increases, the patent says, the charge and discharge cycle life increases although the relationship is said not to be linear. That is, in a pressure range of from 40 to 60 g/dm.sup.2, the life is abruptly increased by a factor of 2 to 2.5 as the pressure increased. Therefore, up to about 100 kg/dm.sup.2, the life remains substantially unchanged. However, if pressure exceeds about 100 kg/dm.sup.2, the life decreases. The tendency to decrease depends on the type of glass mat used. The life of the lead acid cell using the glass mat having a dual layer structure was found to be excellent at a low pressure range while the life of such cell increases by a fator of about two at a pressure of 20 kg/dm.sup.2.
The patent also states that the pressure applied to the assembled element presses on each plate and prevents the aforementioned expansion effect attributed to changes in structure of the active material layer. During use, it is necessary to prevent a reduction of the degree of pressure. It was found that the greatest cause for the reduction of the pressure applied to the assembled element is that, when the glass mat is wetted, its thickness decreases. The patent discloses that the degree of reduction of pressure applied to the glass mat depends upon the technique used in fabricating the glass mat. In general it is stated that it is desirable in a glass mat employed in a lead acid battery for the degree of pressure applied when the mat is immersed in dilute sulfuric acid to be more than 70% of the degree of pressured applied in the dry state. The importance of pressure was clearly noted and the main solution was external pressure devices. The method of making the glass material was not stated but a Japanese disclosure, No. 5505306 JPA1 issued to applicant: Japan Storage Battery Company Ltd., discloses a dual layer glass mat produced by a wet laid process. U.S. Pat. No. 4,336,314 refers to a Japanese Patent Office publication number, 55091564 JP A1, which is said to have a date of publication of Jul. 11, 1980.
U.S. Pat. No. 4,363,856 (Waterhouse) discloses a separator material made from a furnish composed of polyolefin pulp fibers and glass fibers, and names polyester staple fibers, polyolefin staple fibers and cellulose pulp fibers as alternative constituents of the furnish.
U.S. Pat. No. 4,387,144 (McCallum) discloses a battery separator having a low electrical resistance after extended use which is made by thermal consolidation and thermal embossing of a paper web formed from a furnish containing a synthetic pulp composed of fibrils which are filled with an inorganic filler, the web incorporating a wetting agent which is preferably an organic sulfonate, an organic succinate, or a phenol ethoxylate.
U.S. Pat. No. 4,373,015 (Peters et al.) discloses sheet material for use as a separator in a battery, and "comprising organic polymeric fibers"; both of the examples of the reference describe the sheet material as "short staple fiber polyester matting about 0.3 mm thick", and indicate that the polyester fibers range from about 1 .mu.m to about 6 .mu.m in diameter.
Sheet separators for use in conventional (not valve regulated) batteries and comprising both glass fibers and organic fibers are disclosed in all of the following U.S. Pat. No. 4,529,677 (Bodendorf); U.S. Pat. No. 4,363,856 (Waterhouse); and U.S. Pat. No. 4,359,511 (Strzempko).
U.S. Pat. No. 4,367,271, Hasegawa, discloses storage battery separators composed of acrylic fibrils in an amount of up to about 10 percent w/w, balance glass fibers.
Japanese patent document 55/146,872 discloses a separator material comprising glass fibers (50-85 percent w/w) and organic fibers (50-15 percent w/w).
U.S. Pat. No. 4,245,013, Clegg et al., discloses a separator made by overlaying a first sheet of fibrous material including polyethylene fibers with a second sheet of fibrous material including polyethylene and having a synthetic pulp content higher than the first sheet.
U.S. Pat. No. 5,009,971, Johnson et al., discloses a porous flexible sheet of about 93 to 99.5 weight percent amorphous precipitated silica and from about 0.5 to about 7 weight percent fibrillated, unsintered polymeric material, e.g., polytetrafluoroethylene. The sheet is prepared by subjecting a dry homeogeneeous mixture of the silica and polymeric material, e.g., polytetrafluoroethylene, in the above proportions to mechanical shear blending forces to finrillate the polymer, and thereafter dry forming the resulting admixture into a sheet form.
U.S. Pat. No. 4,908,282, Badger, discloses a separator comprising a sheet made from first fibers which impart to the sheet an absorbency greater than 90% and second fibers which impart to the sheet an absorbency less than 80% wherein the first and second fibers are present in such proportions that the sheet has an absorbency of from 75 to 95%. This patent discloses that fine glass fibers have a high absorbency, that coarse glass fibers have a low absorbency, and that hydrophobic organic fibers have an extremely low absorbency, and that, when this separator is saturated with electrolyte, unfilled voids remain so that gas can transfer from plate to plate for recombination. The disclosure of Badger is incorporated herein by reference.
U.S. Pat. No. 5,091,275 (Brecht et al.) discloses a glass fiber separator which expands when exposed to electrolyte. The separator comprises glass fibers which are impregnated with an aqueous solution of colloidal silica particles and a sulfate salt. The separator is produced by forming a paper making web of glass fibers, impregnating the web with the aqueous mixture of silica and the salt, lightly compressing the impregnated web to remove some of the aqueous solution, partially drying the web, compressing the web to a final thickness and completing the drying of the web. The web is preferably compressed to a thickness which is less than the distance between plates in a given cell, so that insertion of an assembled cell stack into a case is facilitated. When electrolyte is added to the case, the salt dissolves in the electrolyte and the separator expands to provide good contact between the plates and the separators. According to the patent, the silica contributes to the recombination performance of cells incorporating the precompressed separator. The silica also contributes a great deal of stiffness to the separator, so much so that the separator may be characterized as rigid.
It has been determined that the production of battery separator by paper-making techniques from a furnish of glass fibers and silica powder leads to problems which are caused by variations in the concentration of the silica powder in the furnish. Typical glass fiber furnishes have a liquid content exceeding 98 percent w/w. In the course of making separator sheets, the furnish flows from a headbox onto an advancing screen through which most of the water flows in the first few feet. The water, known as white water, is recycled and winds up back in the headbox of the machine. If the furnish is composed exclusively of glass fibers, virtually none of the fibers pass through the wire and wind up in the white water. However, furnishes comprising glass fibers and silica powder do not fare so well. In the absence of a retention aid, significant amounts of silica powder from such furnishes do pass through the paper making wire and wind up in the white water. Left unchecked, this phenomenon causes the concentration of silica powder in the furnish to increase, undesirably changing the properties of the furnish. Heretofore, the problem of silica powder and the like passing through a paper making wire has been avoided through the use of binders and retention aids.
U.S. Pat. No. 2,477,000 discloses a synthetic fiber paper produced from fibrillae and fibers made by methods wherein a solution of the fiber is extruded through very small orifices (spinnerets).and the extruded solution is allowed to congeal in a precipitating bath, by evaporation of the solvent, or by temperature changes (see column 2, lines 25 and following). The patent says that fibers of cellulose acetate, cellulose nitrate, regenerated cellulose from viscose, "Vinylite (a synthetic resin made by polymerization of vinyl compounds), Aralac (a fibrous product made from skim milk casein), and spun glass" which range in length up to 1 inch and in diameter from 12-80 microns and fibrillae preferably derived from flax, Manila hemp, caroa or hemp can be used to make the paper. At least 90 percent of the fibrillae should be from 0.0015 to 0.0025 inch in length and from 0.0000027 to 0.0000044 inch in width.
WO 98/12759, an International Application published Mar. 26, 1998, discloses "A resilient fibrous mat, preferably made of microfibers (which) is especially adapted for use as a battery separator for starved electrolyte batteries . . . The fibrous mat, with one or two surface layers, can be formed from an air laid fibrous blanket by subjecting one or both surfaces of the blanket to hydro entanglement to increase the entanglement of the fibers at and adjacent the major surface(s) relative to the entanglement of the fibers in the resilient fibrous layer. The fibrous mat with a substantially uniform density may be made by flooding the blanket with a liquid and drawing a vacuum through the blanket."
European Patent Application 97116846, Japan Vilene Co., Ltd., filed Sep. 29, 1997, published as EP 0834932 A2, shows the kind of entanglement disclosed in WO98/12759 to produce the material of FIGS. 1 and 2 thereof, but applied to the entire body of the separator material rather than to a region or regions adjacent one or both major surfaces as in WO98/12759.
An English language abstract of a published Japanese patent application (07147154, published Jun. 6, 1995), entitled SEPARATOR FOR ALKALINE BATTERY states:
"A fiber having a section form shown in (c) of the drawing, for example, is constituted from 0.04 to 0.12 deniers of circular and petal polypropylene component 2 and 0.12 denier of a polyethylene component 1. A hundred percent of this dividing composite fiber with a fineness of 2 deniers and a fiber length of 38 mm is opened by a card machine to laminate unidirectional and cross fiber webs with METSUKE of 1.3 and 52 f/m.sup.2. This is treated from both surfaces with a water flow having a water pressure of 130 kg/cm.sup.2 on a nozzle plate having a nozzle diameter of 0.13 mm and a pitch of 0.6 mm. This cloth is dipped in fuming sulfuric acid, sulfonated, and then calendered to provide a separator having a METSUKE of 65 g/m.sup.2 and a thickness of 0.15 mm. The same treatment can be performed in constitutions other than (c) in the drawing. Thus, excellent electrolyte resistance, oxidizing property, and liquid holding property are provided, and a battery can be smoothly operated for a long period."